Posture

Posture is the alignment of the body parts, whether upright, sitting, or recumbent. It is described by the positions of the joints and body segments, and also in terms of the balance between the muscles crossing the joints. Impairments in the joints, muscles, or connective tissues may lead to faulty postures, or, conversely, faulty postures may lead to impairments in the joints, muscles, and connective tissues as well as symptoms of discomfort and pain. Many musculoskeletal complaints can be attributed to stresses that occur from repetitive or sustained activities when in a habitually faulty postural alignment.

Structure and Function of the Spine

Structure

The structure of the spinal column consists of 33 vertebrae (7 cervical, 12 thoracic, 5 lumbar, 5 fused sacral, and 3 or 4 coccygeal) and their respective intervertebral discs. Articulating with the spine are the 12 pairs of ribs in the thoracic region, the cranium at the top of the spine at the occipital-atlas joint, and the pelvis at the sacroiliac joints.

Functional Components of the Spine

Functionally, the spinal column is divided into anterior and posterior pillars.

Anterior Pillar

The anterior pillar is made up of the vertebral bodies and intervertebral discs and is the hydraulic, weight-bearing, shock-absorbing portion of the spinal column. The size of the disc influences the amount of motion available between two vertebrae.

Posterior Pillar

The posterior pillar, or vertebral arch, is made up of the articular processes and facet joints, which provide the gliding mechanism for movement. The orientation of the facets influences the direction of motion. Also part of the posterior unit are the boney levers, the two transverse processes, and the spinous process to which the muscles attach and function to cause and control motions and provide spinal stability.

Motions of the Spinal Column

Motion of the spinal column is described both globally and at the functional unit or motion segment. The functional unit is comprised of two vertebrae and the joints in between (typically, two zygapophyseal facet joints and one intervertebral disc). Generally, the axis of motion for each unit is in the nucleus pulposus of the intervertebral disc. Because the spine can move from top down or bottom up, motion at a functional unit is defined by what is occurring with the anterior portion of the body of the superior vertebra.

The Six Degrees of Motion

Flexion/extension

Motion in the sagittal plane results in flexion (forward bending) or extension (backward bending). With flexion, the anterior portions of the bodies approximate and the spinous processes separate; with extension, the anterior portions of the bodies separate and the spinous processes approximate.

Side bending

Motion in the frontal plane results in side bending (lateral flexion) to the left or right. With side bending, the lateral edges of the vertebral bodies approximate on the side toward which the spine is bending and separate the opposite side.

Rotation

Motion in the transverse plane results in rotation. Rotation to the right results in relative movement of the body of the superior vertebrae to the right and its spinous process to the left; the opposite occurs with rotation to the left. If movement occurs from the pelvis upward, the motion is still defined by the relative motion of the top vertebra.

Anterior/posterior shear.

Forward or backward shear(translation) occurs when the body of the superior vertebra translates forward or backward on the vertebra below.

Lateral shear.

Lateral shear (translation) occurs when the body of the superior vertebra tr anslates sideways on the vertebra below.

Compression/distraction.

Separation or approximation occurs with a longitudinal force, either away from or toward the vertebral bodies.

Arthrokinematics of the Zygapophyseal (Facet) Joints

Each region of the spine has its own special considerations as pertains to ar throkinematic movement and function. The remainder of the cervical spine and all the thoracic facets have relatively flat articular surfaces and glide on the adjacent facet joint. is The superior facets of the lumbar spine are concave and articulate with the adjacent inferior convex facets.

Coupled motions typically occur at a segmental level when a person side bends or rotates their spine. Coupled motion is defined as the consistent association of one motion about an axis with another motion around a different axis and varies depending on the region, the spinal posture, the orientation of the facets, and factors such as the extensibility of the soft tissues. When motions of side bending and rotation are coupled, foraminal opening is dictated by the side bending component.

Cervical spine

The cervical spine can be divided into the craniovertebral region and the typical cervical region.

The criniovertebral region is composed of the occiput, atlas, and superior facets of the axis.

The occipito-atlanto (OA) joint is considered a ball and socket joint; the convex facets of the occiput articulate with the concave facets of the atlas. Its pr imary motions are forward and backward nodding (flexion and extension). There is a small amount of side bending avail-able at the OA joint; rotation and side bending are coupled in opposite directions in this region.

The atlanto-axial (AA) joint consists of convex articulating surfaces of the atlas articulating on the convex articulating surfaces of the axis; its primary motion is rotation as the atlas pivots around the dens of the axis. It is important to note that, during rotation, one side of the AA joint complex is behaving as though it is flexing (moving forward) and the other side as though it is extending (moving backward).

The typical cervical region includes the inferior facets of the axis and rest of the cervical spine; it features facet joints that are angled at 45° from the horizontal plane. Side bend-ing and rotation typically couple toward the same side.

Another unique characteristic of the cervical spine is the joints of Luschka. These bony projections provide lateral stability to the spine and reinforce the vertebral disc posterolaterally.

Thoracic spine.

The thoracic facets begin in a frontal plane orientation and transition to a sagittal plane orientation as they near the lumbar spine. The ribs articulate with the thoracic spine at the transverse processes as well as the vertebral bodies and IV discs. In the upright posture, side bending and rotation typically couple in the same direction in the upper thoracic spine and in the opposite directions in the lower thoracic region, although variability has been described.

Lumbar spine.

As the lumbar facets transition from a sagit-stal plane to a frontal plane orientation, some of the facets have a biplanar orientation. Coupling varies in that with lateral flexion, rotation occurs to the same side, but with rotation, lateral flexion occurs opposite, there is variability with flexion and extension.

Structure and Function of Intervertebral Discs and Cartilaginous End-Plates

The intervertebral disc, consisting of the annulus fibrosus and nucleus pulposus, is one component of a three-joint complex between two adjacent vertebrae and cartilaginous end-plates of the vertebral bodies. The structure of the disc dictates its function.

Annulus fibrosus.

The outer portion of the disc is made up of dense layers of type I collagen fibers. The collagen fibers in any one layer are parallel and angled around 60° to 65" to the axis of the spine, with the tilt alternating in successive layers. Because of the orientation of the fibers, tensile strength is provided to the disc by the annulus when the spine is distracted, rotated, or bent. This structure helps restrain the various spinal motions as a complex ligament. The annulus is firmly attached to adjacent vertebrae, and the layers are firmly bound to one another. Fibers of the innermost layers blend with the matrix of the nucleus pulposus. The annulus fibrosus is supported by the anterior and posterior longitudinal ligaments.

Nucleus pulposus.

The central portion of the disc is a gelatinous mass that normally is contained within, but whose loosely aligned type II collagen fibers merge with the inner layer of the annulus fibrosus. It is located centrally in the disc except in the lumbar spine, where it is situated closer to the posterior border than the anterior border of the annulus. Aggregating proteoglycans, normally in high concentration in a healthy nucleus, have great affinity for water. The result-ing fluid mechanics of the confined nucleus functions to distribute pressure evenly throughout the disc and from one vertebral body to the next under loaded conditions. Because of the affinity for water, the nucleus imbibes water when pressure is reduced on the disc and squeezes water out under com-pressive loads. These fluid d ynamics provide tr ansport for nutrients and help maintain tissue health in the disc.

With flexion (forward bending) of a vertebral segment, the anterior portion of the disc is compressed, and the posterior is distracted. The nucleus pulposus generally does not move in a healthy disc but may have slight distortion with flexion, potentially to redistribute the load through the disc Asym-metrical loading in flexion results in distortions of the nucleus toward the contralateral posterolateral corner, where the fibers of the annulus are more stretched.

Cartilaginous end-plates.

End-plates cover the nucleus pulposus superiorly and inferiorly and lie between the nucleus and vertebral bodies. Each is encircled by the apophy-sealr ing of the respective vert ebral body. The collagen fibers of the inner annulus fibrosus insert into the end-plate and angle centrally, thus encapsulating the nucleus pulposus. Nutrition diffuses from the marrow of the vertebral bodies to the disc via the end-plates. The end-plates are also responsible for containing the nucleus from migrating superior/inferior.

Intervertebral Foramina

The intervertebral foramina are between each vertebral segment in the posterior pillar. Their anterior boundary is the intervertebral disc, the posterior boundary is the facet joint, and the superior and inferior boundaries are the pedicles of the superior and inferior vertebrae of the spinal segment. The mixed spinal nerve ex its the spinal canal via the foramen along with blood vessels and recurrent meningeal or sinuver-tebral nerves. The size of the intervertebral foramina is af-fected by spinal motion, being larger with forward bending and contralateral side bending and smaller with extension and ipsilateral side bending.

Biomechanical Influences on Postural Alignment

Curves of the Spine

The adult spine is divided into four curves: two primary, or posterior, curves, so named because they are present in the infant and the convexity is posterior, and two compensatory, or anterior, curves, so named because they develop as the infant learns to lift the head and eventually stand, and the convexity is anterior.

Posterior curves are in the thoracic and sacral regions.

Kyphosis is a termused to denote a posterior curve. Kyphotic posture refers to an excessive posterior curvature of the thoracic spine.

Anterior curves are in the cervical and lumbar regions.

Lordosis is a term also used to denote an anterior curve, although some sources reserve the term lordosis to denote abnormal conditions such as those that occur with a sway back.46

The curves and flexibility in the spinal column are important for withstanding the effects of gravity and other external forces.

The structure of the bones, joints, muscles, and inert tissues of the lower extremities are designed for weight bearing; they support and balance the trunk in the upright posture.

Gravity

When looking at posture and function, it is critical to under-stand the influence of gravity on the structures of the trunk and lower extremities. Gravity places stress on the structures responsible for maintaining the body upright and therefore provides a continual challenge to stability and efficient move-ment. For a weight-bea ring joint to be stable, or in equilibrium, the gravity line of the mass must fall exactly through the axis of rotation, or there must be a force to counteract the moment caused by gravity. In the body, the counterforce is provided by either muscle or inert structures. In addition, the standing posture usually involves a slight anterior/posterior swaying of the body of about 4 cm, so muscles are necessary to control the sway and maintain equilibrium.

In the upright posture, the line of gravity transects the spinal curves, which are balanced anteriorly and posteriorly, and it is close to the axis of rotation in the lower extremity joints. The following describes the standard of a balanced upright posture.

Ankle

For the ankle, the gravity line is anterior to the joint, so it tends to rotate the tibia forward about the ankle. Stability is provided by the plantarflexor muscles, primarily the soleus muscle.

Knee

The normal gravity line is anterior to the knee joint, which tends to keep the knee in extension. Stability is provided by the anterior cruciate ligament, posterior capsule (locking mechanism of the knee), and tension in the muscles posterior to the knee (the gastrocnemius and hamstring muscles). The soleus provides active stability by pulling posteriorly on the tibia. With the knees fully extended, no muscle support is required at that joint to maintain an up right posture; how-ever, if the knees flex slightly, the gravity line shifts posterior to the joint, and the quadriceps femoris muscle must contract to prevent the knee from buckling.

Hip

The gravity line at the hip varies with the swaying of the body. When the line passes through the hip joint, there is equilibrium, and no external support is necessary. When the gravitational line shifts posterior to the joint, some posterior rotation of the pelvis occurs but is controlled by tension in the hip flexor muscles (primarily the iliopsoas). During relaxed standing, the iliofemoral ligament provides passive stability to the joint, and no muscle tension is necessary. When the gravitational line shifts anteriorly, stability is provided by active support of the hip extensor muscles.

Trunk

Normally, the gravity line in the trunk goes through the bodies of the lumbar and cervical vertebrae, and the curves are balanced. Some activity in the muscles of the trunk and pelvis helps maintain the balance. (This is described in greater detail in the following sections.) As the trunk shifts, contralat-eral muscles contract and function as guy wires. Extreme or sustained deviations are supported by inert structures.

Head

The center of gravity of the head falls anterior to the atlanto-occipital joints. The posterior cervical muscles contract to keep the head balanced.

Stability

When standing, the center of gravity ty pically falls slightly anterior to S2 in the pelvis. So long as the line of gravity from the center of mass falls within the base of support, a structure is stable. Stability is improved by lowering the center of grav ity or increasing the base of support. In the upright position, the body is relatively unstable because it is a tall structure with a small base of support. When the center of gravity falls out-side the base of support, either the structure falls or some force must act to keep the structure cture upright. Both inert and dynamic structures support the body against gravitational and other external forces. The inert osseous and ligamentous structures provide passive tension when a joint reaches the end of its range of motion (ROM). Muscles act as dynamic guy wires, responding to perturbations by providing coun-terforces to the torque of gravity as well as stability within the ROM so stresses are not placed on the inert tissues.

Postural Stability in the Spine

Spinal stability is described in terms of three subsystems: passive (inert structures/bones and ligaments), active (muscles), and neural control. The three subsystems are interrelated and can be thought of as a three-legged stool; if any one of the legs is not providing support, it affects the stability of the whole structure. Instability of a spinal segment is often a combination of inert tissue damage, insufficient muscular strength or endurance, and poor neuromuscular control.

Inert Structures: Influence on Stability

Penjabi described the ROM of any one segment as being divided into an elastic zone and a neutral zone. When spinal segments are in the neutral zone (midrange/neutral range), the inert joint capsules and ligaments provide minimal passive resistance to motion and therefore minimal stability. As aseg-ment moves into the elastic zone, the inert structures provide restraint as passive resistance to the motion occurs. When a structure limits movement in a specific direction, it provides stability in that direction. In addition to the inert tissues pro-viding passive stability when limiting motion, the sensory receptors in the joint capsules and ligaments sense position and changes in position. Stimulation of these receptors provides feedback to the central nervous system, thus influencing the neural control system.

Muscles: Influence on Stability

The muscles of the trunk not only act as prime movers or as antagonists to movement caused by gravity during dynamic activity; they are important stabilizers of the spine. Without the dynamic stabilizing activity from the trunk mus-cles, the spine would collapse in the upright position.

Role of Global and Segmental Muscle Activity

Both superficial (global) and deep (segmental) muscles play critical roles in providing stability and maintaining the upright posture.

Global muscle function.

In the lumbar spine, the global mus-cles, being the more superficial of the two groups, are the large Buy wires that respond to extemal loads imposed on the trunk that shift the center of mass. Their reaction is direction specific to control spinal orientation. The global muscles are unable to stabilize individual spinal segments except through compressive loading because they have little or no direct attachment to the vertebrae. If an individual segment is unstable, compressive loading from the global guy wires may lead to or perpetuate a painful situation as stress is placed on the inert tissues at the end of the range of that segment.

Deep/segmental muscle function.

The deeper, segmental muscles, which have direct attachments across the vertebral segments, provide dynamic support to individual segments in the spine and help maintain each segment in a stable position, so the inert tissues are not stressed at the limits of motion.

Muscle Control in the Lumbar Spine

Abdominal muscles. The rectus abdominis (RA), external oblique (EO), and internal oblique (IO) mus-cles are large, multisegmental global trunk flexor muscles and are important guy wires for stabilizing the spine against postural perturbations. The transversus abdominis (TrA) is the deepest of the abdominal musdes and responds uniquely to postural perturbations. It attaches posteriorly to the lumbar vertebrae via the posterior and middle layers of the thoracolum-bar fascia and through its action develops tension that acts like a girdle of support around the abdomen and lumbar vertebrae. Only the TrA is active with both isometric trunk flexion and extension, whereas the other abdominal muscles have decreased activity with resisted extension. This is attributed to the stabilization function of the TrA.

Transversus abdominis stabilization activity.

Early electromyographic research studies of the activity of the deeper abdominal muscles in their stabilization function were done with surface electrodes and did not discriminate activity between the TrA and IO. Byusing ultrasound imaging techniques, insertion of fine-needle electrodes into the various muscles has produced evidence of differing functions between these two muscles with perturbations to balance in healthy individuals as well as those who have low back pathology.

The TrA responds with anticipatory activity and with rapid arm and leg movements (before the other abdominals) and coordinates with respiration during these activities. The TrA also has a coordinated link with the perineum and pelvic floor muscle function as well as with the deep fibers of the multifidi. The drawing in maneuver is used to activate the TrA voluntarily and, with training, produces the most independent activity of this muscle. Training the TrA for postural control and stability has been shown to improve the long term outcome in patients experiencing their first episode of low back pain.

Erector spinae muscles

The erector spinae muscles are the long, multisegmental extensors that begin as a large musculotendinous mass over the sacral and lower lumbar vertebrae. They are important global guy wires for controlling the trunk against postural perturbations.

Multifidus stabilization activity.

The multifasciculed multifidi muscle group has a high distribution of type I fibers and large capillary network, emphasizing its role as a tonic stabi-lizer. Itssegmental attachments are able to control movement of the spinal segments as well as increase spinal stiffness. The multifidus, along with the erector spinae, are encased by the posterior and middle layers of the lumbodorsal fascia, so bulk and muscle contraction increase tension on the fascia, adding to the stabilizing function of the fascia (see below for a description of this mechanism).

In patients with low back impairment, the fibers of the multifidi quickly atrophy at the spinal segment, and a moth-eaten appearance has been reported in patients undergo ing surgery for lumbar disc disease. 66 Additionally, it has been reported that individuals with LBP have been shown to have significantly more fatty infiltration in the LM when compared with healthy controls. This reflects that muscle quality might be a precursor in reoccurring LBP. Evidence supports the idea that training with specific exercises increases the func-tion of the multifidi as well as the erector spinae in gen-eral. Other deep muscles that theoretically play a role in segmental stability but to this point in time have been difficult to assess because of their depth include the intersegmental muscles (rotators and intertransversarii muscles) and deep fibers of the quadratus lumborum.

Thoracolumbar (lumbodorsal) fascia.

The thoracolumbar fascia is an extensive fascial system in the back that consists of several layers. It surrounds the erector spinae, mul-tifidi, and quadratus lumborum, thus providing support to these muscles when they contract. Increased bulk in these muscles increases tension in the fascia, perhaps contributing the stabilizing function of these muscles. The aponeurosis of the latissimus dorsi and fibers from the serratus posterior inferior, internal oblique, and transverse abdominis muscles blend together at the lateral raphe of the thoracolumbar fascia, so contraction in these muscles increases tension through the angled fascia, providing stabi-lizing forces for the lumbar spine. In addi-tion, the X design of the latissimus dorsi and contralateral gluteus maximus has the potential to provide stability to the lumbosacral junction.

Muscle Control in the Cervical Spine

The fulcrum of the head on the spine is through the occipital/atlas joints. The center of gravity of the head is anterior to the joint axis and therefore has a flexion moment. The weight of the head is counterbalanced by the cervical extensor muscles (upper trapezius and cervical erector spinae). Tension and fatigue in these muscles, as well as in the levator scapulae (which supports the posture of the scapulae), is experienced by most people who experience postural stress to the head and neck. The position of the mandible and the tension in the muscles of mastication are influenced by the postural relationship between the cervical spine and head.

Mandibular elevator group.

The mandible is a movable structure that is maintained in its resting position with the jaw partially closed through action of the mandibular eleva-tors (masseter, temporalis, and medial pterygoid muscles).

Suprahyoid and infrahyoid group.

The anterior throat muscles assist with swallowing and balancing the jaw against the muscles of mastication. These muscles also function to flex the neck when rising from the supine position. With a forward head posture, they, along with the longus colli, tend to be stretched and weak so the person lifts the head with the sternocleidomastoid (SCM) muscles. In addition, with a for-ward head posture, the suprahyoid muscles have a tendency to pull the mandible into depression due to their orientation and attachments at the hyoid and mandible. This is counter-acted by the mandibular elevator muscle group, which creates a sustained contraction in order to keep the mouth closed.

Rectus capitis anterior and lateralis, longus colli, and longus capitis.

The deep craniocervical flexor muscles have segmental attachments and provide dynamic support to the cervical spine and head. The longus colli is important in the action of axial extension (retraction) and works with the SCM for cervical flexion. Without the seg mental influence of the longus colli, the SCM would cause increased cervical lordosis when attempting flexion.

Multifidus.

With its segmental attachments, the multifidus is thought to have alocal stabilizing function in the cervical spine similar to its function in the lumbar region.

Role of Muscle Endurance

Strength is critical for controlling large loads or responding to large and unpredictable loads (such as during laborious activities, sports, or falls), but only about 10% of maximum contraction is needed to provide stability in usual situations, Slightly more might be needed in a segment damaged by disc disease or ligamentous laxity when muscles are called on to compensate for the deficit in the passive support. In a study that looked at mechanical factors and the occurrence of low back pain in 600 subjects, poor muscular endurance in the back extensors muscles had the greatest association with low back pain.

Greater percentages of type I fibers than type II fibers are found in all back muscles, which is reflective of their postural and stabilization functions,

Neurological Control: Influence on Stability

The muscles of the neck and trunk are activated and con-trolled by the nervous system, which is influenced by periph-eral and central mechanisms in response to fluctuating forces and activities. Basically, the nervous system coordinates the response of muscles to expected and unexpected forces at the right time and by the right amount by modulating stiffness and movement to match the various imposed forces.

Feedforward control and spinal stability.

The central nervous system activates the trunk muscles in anticipation of the load imposed by limb movement to maintain stability in the spine, Research has demonstrated that there are feedforward mechanisms that activate postural responses of all trunk muscles preceding activity in muscles that move the extremities and that anticipatory activation of the transversus abdominis and deep fibers of the multifidus is independent of the direction or speed of the postural disturbance. The more superficial trunk muscles vary in response depending on the direction of arm and leg movement, reflective of their postural guy wire function, which controls displace-ment of the center of mass when the body changes config-uration. There are reported differences in patterns of muscle recruitment in patients with low back pain with delayed recruitment of the transversus abdominis in all movement directions and delayed recruitment of the rectus abdominis, erector spinae, and oblique abdominal muscles specific to the direction of movement compared to healthy subjects.

Effects of Limb Function on Spinal Stability

Without adequate stabilization of the spine, contraction of the limb-girdle musculature transmits forces proximally and causes motions of the spine that place excessive stresses on spinal structures and the supporting soft tissue.

CLINICAL TIP

Stabilization of the pelvis and lumbar spine by the ab-dominal muscles against the pull of the iliopsoas muscle is necessary during active hip flexion to avoid increased lumbar lordosis and anterior shearing of the vertebrae.

Stabilization of the ribs by the intercostal and abdominal muscles is necessary for an effective pushing force from the pectoralis major and serratus anterior muscles.

Stabilization of the cervical spine by the longus colli muscle is necessary to prevent excessive lordosis from contraction of the upper trapezius as it functions with the shoulder girdle muscles in lifting and pulling activities.

Localized muscle fatigue.

Localized fatigue in the stabilizing spinal musculature may occur with repetitive activity or heavy exertion or when the musculature is not utilized effectively due to faulty postures. There is a greater chance of injury in the supporting structures of the spine when the stabilizing muscles fatigue. Marras and Granata reported significant changes in motion patterns between the spine and lower extremity joints as well as significant changes in muscle recruit-ment patterns with repetitive lifting during an extended period of time, resulting in increased anterior/posterior shear in the lumbar spine.

Muscle imbalances.

Imbalances in the flexibility and strength of the hip, shoulder, and neck musculature cause asymmetrical forces on the spine and affect posture.

Effects of Breathing on Posture and Stability

Inspiration and thoracic spine extension elevate the rib cage and assist with posture. The intercostal muscles function as postural muscles to stabilize and move the ribs. They act as a dynamic membrane between the ribs to prevent sucking in and blowing out of the soft tissue with the pressure changes during respiration. The stabilizing function of the TrA also works in conjunction with the diaphragm in a feed-forward response to rapid arm motions. Contraction of the diaphragm and increased intra-abdominal pressure (IAP) occur prior to rapid arm movement, irrespective of the phase of respiration or the direction of the arm motion. The tonic activities of the TrA and diaphragm are modulated to meet respiratory demands during both inspiration and expiration and provide stability to the spine during repetitive limb movements.

The Spine, Structure, Function and Posture